1. Field of the Invention
This invention relates to an LED head for forming a dot on a photosensitive drum by focusing a light beam from a row of light-emitting elements at a predetermined position by a condenser lens, and more particularly to an arrangement for setting a distance between the row of light-emitting elements and the photosensitive drum and for securing a positional relationship between the row of light-emitting elements and the condenser lens.
2. Description of Related Art
An LED printer is a printer for printing an image formed on a photosensitive drum by an LED head. The LED head is a head which comprises a plurality of light-emitting diodes arranged in a row and a condenser lens disposed between the row of light-emitting diodes and the photosensitive drum. The condenser lens converges a light beam from the row of light-emitting diodes to focus it on the surface of the photosensitive drum. If the row of light-emitting diodes, the condenser lens and the photosensitive drum are positioned in a precise arrangement, it is possible to achieve a good printing quality.
FIG. 8 of the accompanying drawings is a schematic side view of a focusing mechanism for setting the distance between a row of light-emitting diodes and a photosensitive drum 14 to a predetermined value, illustrating a typical conventional LED head.
The row of light-emitting diodes and an integrated circuit for activating it are supported on a circuit board 10. In FIG. 8, parts or elements on the circuit board 10 are not seen as they are covered by a head cover 16 (described below). A condenser lens row 12 is disposed between the light-emitting-diode-row-supporting surface of the circuit board 10 and the photosensitive drum 14. The condenser lens row 12 includes a plurality of condenser lenses formed as a unit in association with the individual light-emitting diodes for focusing an output light beam of each light-emitting diode onto the surface of the photosensitive drum 14. The head cover 16 is mounted on the light-emitting-diode-row-supporting surface of the circuit board 10, covering the row of light-emitting diodes. The condenser lens row 12 is fixed to the upper surface of the head cover 16, so the head cover 16 serves to set the distance between the circuit board 10 and the condenser lens row 12 to a predetermined value.
A heat-radiating plate 18 is attached to the lower surface of the circuit board 10. The heat-radiating plate 18 serves to assist in removing heat generated by parts or elements on the circuit board 10 and constitutes a positioning mechanism 20 for positioning the light-emitting diode row with respect to the photosensitive drum 14. The positioning mechanism 20 includes a pin 22 projecting from the heat-radiating plate 18, and a support post 24 leading from one end surface of the photosensitive drum 14 and having in one end a recess 26 engageable with a distal end of the pin 22.
In this conventional arrangement, the distance between the circuit board 10 supporting the light-emitting diode row and the photosensitive drum 14 is set with the upper surface of the heat-radiating plate 18 as a reference surface. In this case, the varying thickness of the circuit board 10 would be the cause for reduced precision in the distance between the circuit board 10 and the photosensitive drum 14.
FIG. 9 is a schematic side view of another conventional LED head having a positioning mechanism 28 different from that of FIG. 8. In FIG. 9, parts or elements similar to those of FIG. 8 are designated by like reference numerals.
In this positioning mechanism 28, the upper surface of a head cover 30 is a reference surface. The head cover 30 and the circuit board 32 extend toward and terminate immediately short of one end of the heat-radiating plate 18, and the head cover 30 has in its upper surface a recess 34. A support post 36 leading from the end surface of the head cover 30 has a rounded distal end engageable with the recess 30.
This arrangement causes the same problem as with other conventional art. Namely, since the reference surface is the upper surface of the head cover, the varying thickness of the head cover 30 would cause reduced precision in the distance between the circuit board 32 and the photosensitive drum 14.
Another problem of the conventional art is that the precision in relative position of the light-emitting diode row and the condenser lens row would also be deteriorated.
FIG. 10 is an exploded perspective view of another LED head illustrating the last-mentioned problem. In FIG. 10, the parts or elements similar to those of FIG. 8 are designated by like reference numerals. Assuming that the positional relationship between the light-emitting diode row arranged on the circuit board 10 and the condenser lens row 12 fixed on the head cover 16 is determined properly, light emitted from the individual light-emitting diode will be focused correctly on the photosensitive drum by the corresponding condenser lens. In the conventional art, the positional precision is secured by means of a reference pin 38 and a through-hole 40. The reference pin 38 is projecting from the lower surface of the head cover 16, and the through-hole 40 is formed in the circuit board 10. As the reference pin 38 is inserted into the through-hole 40, the positional relationship between the head cover 16 and the circuit board 10 and thus between the condenser lens row 12 and the light-emitting diode row can be determined precisely.
With this conventional art, since the reference pin 38 and the through-hole 40 are needed to secure a precise positional relationship between the light-emitting diode row and the condenser lens row 12, it is inevitable that the process of production would be complex and hence the cost of production would be increased.